As 2020 is almost drawing to a close, we’re still not sure exactly when and how the pandemic started. Now, a new study from the US adds more evidence to the idea that the virus was already circulating outside of China in December, when the Wuhan outbreak was first declared.

Reports of a mysterious pneumonia spreading in Wuhan, China, first emerged in late December 2019 — at least that’s what the official records say. But leaks suggest that China mishandled the early stages of the pandemic and probably even tried to hide the outbreak in its early stages. Governmental data seen by the South China Morning Post indicate that the first COVID-19 cases in China can be traced to November 17, way before the first official Chinese cases.

Now, a new study adds even more controversy, finding that the virus was already circulating in the US in December.

Officially, the first COVID-19 case in the US was reported on January 20. To see if the virus came into the country earlier, researchers analyzed blood donations collected by the American Red Cross from nine states. They found that 106 out of 7,389 blood donations collected from 13 December had coronavirus antibodies: already, a substantial number.

“The presence of these serum antibodies indicate that isolated Sars-CoV-2 infections may have occurred in the western portion of the United States earlier than previously recognised,” CDC scientists wrote in the study.

It’s not the first time something like this has been reported. A study from France also suggested that the virus had reached France since December, while another study found that the virus was circulating in Italy as early as September. This is what Giovanni Apolone, a co-author of the Italian study, told Reuters at the time.

“This is the main finding: people with no symptoms not only were positive after the serological tests but had also antibodies able to kill the virus,” Apolone said.

“It means that the new coronavirus can circulate among the population for long and with a low rate of lethality not because it is disappearing but only to surge again,” he added.

This paints a very murky picture that seems very hard to untangle. We still don’t know when the virus first crossed the animal-human barrier, and what type of animal it came from (though we have some suspects). We also don’t know another important aspect: when the virus started to propagate from human to human. It can take years to unravel these issues, and they may never be truly solved.

However, accurately tracing when a pathogen enters a population is vital for predicting the burden of the disease and for implementing protective measures. For now, one thing’s for sure: the more we look into the early stages of the pandemic, the clearer it becomes that the situation could have been handled much better.

The study has been published in Clinical Infectious Diseases.